The present invention relates to data correction, and in particular, to systems and methods for multistage error correction.
Some communication channels, such as bit patterned recording, suffer from bit insertions and deletions in addition to substitution errors due to loss of synchronization during write or read operations. As used herein, the term “bit insertion/deletion” refers to data values inserted or deleted in a bipolar data stream (+1/−1 or 1/0), which may be a single bit, or sampled data values inserted or deleted in a sampled data stream, where a “bit” insertion/deletion includes insertion or deletion of one bit or multiple bits representing a sample data value.
FIG. 1 illustrates bit insertion and deletion errors due to loss of synchronization during a write operation. FIG. 1 includes scenarios 100-102. Scenario 100 shows an example of bit insertion due to write clock period Tc that is greater than bit-island period Tb. Clock signal 104 has a period Tc which causes an insertion of data bit 3b along side data bit 3a for bit islands 103. Scenario 101 shows an example of a synchronized clock signal 106 that corresponds to having no insertions or deletions for bit islands 105. Clock signal 106 is synchronized with bit islands 105. Scenario 102 shows an example of bit deletion due to write clock signal 108 having a period Tc that is less than bit-island period Tb. The result of the smaller period Tc is a deletion of bit-island 4 of the bit islands 107.
FIG. 2 illustrates an example of a traditional magnetic media 200 and a magnetic bit pattern media 201 used for recording digital data. Magnetic media 200 includes three tracks 203. A track 204 includes a continuous magnetic signature 202 which may be read back and translated into a stream of digital data. Magnetic media 201 has three tracks 207 which are more densely compacted than magnetic media 200. Each of the tracks 207 has a plurality of magnetic islands. Data in a bit pattern media is represented as a pattern of magnetic islands. For instance, magnetic island 205 represents a negative polarized magnetic area, and may be surrounded by other magnetic islands and separated by a space 206 between the islands. Although the bit pattern media 201 may have more densely packed data, the read back data may contain bit insertions and deletions which may cause errors in data synchronization.
Errors in data synchronization may prevent the use of conventional forward error correction (FEC) coding that would benefit from accurate synchronization. In some cases, redundancy may improve the ability to perform error correction, but the more redundant a coding scheme the less dense the actual user data. Current correction schemes, designed for correcting insertions/deletions and substitution errors, typically have low code rate which may be undesirable for many applications.
Thus, it is desirable to improve data correction. The present invention solves these and other problems by providing systems and methods for multistage error correction.